Article: Phylogeny and palaeobiology of Marsupites and Uintacrinus
Clare V. Milsom, Michael J. Simms and Andrew S. Gale
The Upper Cretaceous Marsupites and Uintacrinus are among the morphologically most unusual of all fossil crinoids. Both have a large theca, ten extremely long arms, and lack any anchoring structure in both adult and juvenile stages; this morphology appears so unlike that of other articulate crinoid groups that earlier attempts to identify possible sister groups have been inconclusive. Cladistic analysis indicates that both genera are closely related to the Comasteridae, with Uintacrinus being less derived than Marsupites. Both genera have a virtually world-wide distribution through a limited stratigraphical interval within the Santonian Stage. Their widespread distribution, combined with the absence of a mode of attachment, has led to the conclusion that they were pelagic. However re-examination of their morphology indicates that Marsupites and Uintacrinus were benthic. The global distribution of these taxa probably reflects a long-lived planktotrophic larval stage in the life cycle.